Rectangles in the {Perimeter, Area} plane

The right panel is a {width, height} plane}. Every point in the first quadrant of this plane corresponds to a rectangle.
The applet allows you to generate either a family of rectangles by moving the GOLD point along a
height = constant/width curve or a family of rectangles by moving a GOLD point along a
height+width = constant curve.
[You can position each of these curves by dragging the small WHITE dots.]
Can you explain the nature of the curves generated in the left panel [the {Perimeter, Area} plane] as you drag the GOLD dots in the {width, height} plane? qualitatively? analytically?
What have you learned from this applet?
What questions would/could you put to your students based on this applet?